1. Field of the Disclosure
The present invention relates to a method for recovery of lost and/or corrupted data which is transmitted from a transmitter device to a receiver device.
2. Discussion of the Background Art
The transmitted data can be audio or video streams, for instance. From a transmitter device which makes these data available, the data is transmitted e.g. to a mobile receiver device. The mobile receiver device can be, for instance, a mobile phone, a PDA or another mobile end device. Alternatively, data can also be transmitted from a transmitter device to a stationary receiver device.
Examples of standards used for the transmission of data to mobile end devices include DVB-H, MBMS and, to be expected in the near future, DVB-SH.
In order to guarantee a good transmission quality, it is required to verify the correct transmission of data or data packets to the receiver device. Various methods exist for recovery of lost and/or corrupted data which were not correctly transmitted to the receiver device.
A known method for recovery of lost and/or corrupted data is the Low Density Parity Check (LDPC) method or the Low Density Parity Check Code. This method is applied on a so-called erasure channel. Apart from an application by coding on the level of the physical layer, further applications exist in the field of a Packet Erasure Channel (PEC).
FIG. 1 schematically illustrates an example of the recovery of lost and/or corrupted data according to the state of the art. FIG. 1 depicts a case where it is desired to transmit a number k of information packets from a transmitter device (left-hand side) to a receiver device (right-hand side). Using a packet-level encoder on the transmitter side, the k information packets and the m parity packets will be assembled into n=m+k codeword packets. On the level of the physical layer, the packets are secured by an error correction code (e.g. a turbo code) and an error detection code (e.g. by a Cyclic Redundancy Check, CRC) so that corrupted packets can be removed. On the levels above the physical layer, packets are either correctly received or are considered lost in that they are erased because the CRC has detected a corrupted packet in the physical layer. Thus, from the layers thereabove, the transmission channel is seen as a so-called erasure channel, the packets representing the transmission units. On the receiver side, the received codeword packets are decoded by the packet-level decoder so that the lost and/or corrupted data can be recovered.
The recovery of lost and/or corrupted data can be realized by a redundancy of the data. The encoding process handled by the packet-level encoder is usually performed in a bit-wise (or byte-wise) manner using an encoder with a Generic Binary Linear Block Code. The decoding will subsequently be performed by solving the equation system which is defined by the parity-check matrix H of the code. With increased block lengths, decoders of this type which are based on Gaussian elimination will be of a massively increased complexity so that high data rates can often not be reached.
In principle, the use of a Low Density Parity Check Code as a linear block code will offer two major advantages: The used maximum-likelihood decoder (or the Gaussian elimination) can be replaced by an iterative decoder. This imposes an upper limit on the ability to recover lost and/or corrupted data. Further, for LDPC codes it is possible to simplify the maximum-likelihood decoder by exploiting the sparseness of the parity check matrix.
A reduction of the complexity of the maximum-likelihood decoder does lead to an improved performance but is still relatively complex when compared to the iterative method. Illustrated in FIG. 13 is the performance of a Low Density Parity Check Code with n=1024 and k=512. This Figure shows the development of the Codeword Error Rate (CER), i.e. the decoding errors, when using a maximum-likelihood decoder and an iterative decoder in dependence on the erasure channel probability ε. As a reference curve, the lower bound according to Singleton is represented. The performance of the maximum-likelihood decoder approaches this theoretical limit.
Of considerable importance in a mobile broadcasting application is the capability of packet-level codes to cope with signal fades and outages to the effect that most of the lost and/or corrupted data packets can be restored without a retransmission request. Preferably, use is made of software-implemented packet-level decoders since these do not need a high expenditure for implementation, are easily and flexibly updated and can be adapted by use of terminals which do not need a specific hard-ware design for this purpose. The methods known to date suffer from the disadvantage that either, when using the iterative decoder, it is possible to apply a fast and efficient working decoding method which, however, will yield only poor recovery results, or, when using the maximum-likelihood decoder, the applied method will yield improved recovery results but will have a high complexity and, depending on the given case, a restricted flexibility.
It is an object of the invention to provide a method for recovery of lost and/or corrupted data which are transmitted from a transmitter device to a receiver device, wherein said method shall allow for a better and/or less complex recovery of data.